Advanced Orbiting Systems: A Standard Architecture for Space Data Communications

Abstract

The first thirty years of civilian space exploration were characterized by a series of individual missions, focussed towards specific goals and servicing small and close-knit user communities. Spacecraft (constrained by power, weight and volume considerations) were customized towards mission objectives. Their data handling and communications systems were primarily built for simplicity and robustness, and displayed little commonality from mission to mission. All of the easy space missions have now been flown. As we move into the 1990s, requirements exist for complex missions involving Earth observation, exploration and a more permanent human presence in space. Internationalization of these missions is inevitable as a means to distribute and share costs, and to increase their political stability. Automation of their data handling systems is essential to support reliable, low cost operations. Responding to this environment, the Consultative Committee for Space Data Systems (CCSDS) was formed in 1982 to develop and promote a full suite of internationally standardized space data communications protocols. The first set of recommended standards, covering the data handling requirements of conventional free-flying scientific spacecraft, was finalized in 1986. Using the international space station "Freedom" program (a cooperative venture between the US, Europe, Canada and Japan) as a requirements model, the CCSDS has now extended its suite of recommended standards to cover "advanced orbiting systems" such as unmanned and man-tended Earth observation platforms, new space transportation systems, and manned laboratories. These systems, which operate as longterm orbiting facilities and therefore have changing user communities, produce prodigious rates and volumes of data including digitized video and audio. For the first time, the orbiting systems will use local area networks for internal data transfer. On the ground, they will interface with networks designed for worldwide Open Systems Interconnection (OSI). This paper reviews the standard data handling service architecture which has been developed by CCSDS. It describes the communications protocols that are recommended for the networked transfer of space mission data, and focuses on the unique requirements of transmitting many different data types through weak signal, noisy space channels at rates which routinely may reach many hundreds of megabits per second.